Heh... just multiply by 1hp/.746kw, then 1L/100km is about .43gal/100mi, then invert to get mpg. Easy.
I do like the gas/distance rather than distance/gas as the measure of fuel economy. I find myself doing the "what does it cost for a given trip?" calculation much more often than "how many miles can I go on X gallons?". Plus, that way I can quote my fuel economy in amusing units like acres. (1L/100km = 2.47x10^-12 acre) Or barns. (1L/100km = 100 Exabarns.)